Signal-acquisition device for detecting a high voltage signal at the coils

ABSTRACT

A device is provided for detecting high-voltage characteristics in an ignition system having at least two ignition coils. The device includes a plurality of first capacitive signal couplers each being disposed on a respective one of the ignition coils for capacitively extracting a voltage signal from the ignition coil. A circuit arrangement is connected to the plurality of first capacitive signal couplers and includes an output, and a signal combiner for combining the voltage signals extracted from the ignition coils and producing a combined voltage signal at the output. A plurality of second capacitive signal couplers each connected between the signal combiner and a respective one of the first capacitive signal coupler provides for potential isolation between the plurality of first capacitive signal couplers and a further portion of the circuit arrangement.

BACKGROUND OF THE INVENTION

The invention is based on a device for detecting signals in an ignitionsystem. The ignition system contains two ignition coils, and acapacitive signal coupling provided on the coils.

U.S. Pat. No. 4,795,979 discloses a device for determining the ignitionpulse provided for a specific cylinder of an internal combustion engine.The ignition system contains a plurality of ignition coils, in each casetwo spark plugs being connected to the secondary windings of theignition coils. This distributorless ignition system produces anignition spark at each spark plug at each rotation of the internalcombustion engine. In four-stroke internal combustion engines, theignition sparks are accordingly produced both in the working stroke andin the discharge stroke. For diagnosing this ignition system, threecapacitive signal output couplings are provided which are arranged onthe ignition cables leading from the ignition coils to the spark plugs.The known device requires the ignition cables to be accessible so thatthe sensors can be attached. The extracted signals are fed intodifferent signal evaluation circuits.

U.S. Pat No. 3,959,725 discloses the arrangement of a capacitiveignition voltage sensor on a distributor housing which, in addition tothe distributor, also contains the ignition coil. The capacitive signalextraction is effected with an electrically conductive surface which isattached to the distributor in a suitable manner. This known device isdesigned for the diagnosis of a single-coil ignition system.

SUMMARY OF THE INVENTION

The invention is based on the object of disclosing a simple device witha low cabling outlay for the diagnosis of multi-coil ignition systems.

This object is achieved by a circuit arrangement connected to thecapacitive signal coupling means. The circuit includes an output andmeans for combining the signals extracted from the ignition coils. Thecombined signals are sent to the output. Further, a plurality of secondcapacitive signal coupling means for potential isolation are providedbetween the means for combining and a respective ignition coil.

The device according to the invention for detecting signals in anignition coil system which contains more than one ignition coil has theadvantage of simple adaptation. It is particularly advantageous thatconventional ignition cables no longer have to be present between theignition coils and the spark plugs. The means for the capacitive signalextraction are suitably arranged directly on the ignition coils or onthe housings which contain the ignition coils. The arrangement forcombining the extracted signals, which arrangement is also providedaccording to the invention, reduces the outlay on cabling. It is onlynecessary to have connections between the capacitive signal outputcouplings and the circuit arrangement, from where merely a singleongoing line can be provided. The circuit arrangement is to be arrangedin the direct vicinity of the ignition system so that a short linerouting, in particular of the single lines leading to the capacitivesignal output couplings is possible.

The means for capacitive signal extraction provide signals whichcorrespond in each case to the high voltage at the ignition coils. Thesesignals are combined in the circuit arrangement and can be analysed in afurther diagnostic device. The device according to the invention issuitable for diagnosis in multi-coil ignition systems, the secondarywindings of the ignition coils being wired as desired. For examplesecondary windings which are earthed on one side and have a high-voltageterminal can be provided and also arrangements with spark plug terminalsat both ends of the secondary windings.

A particularly advantageous further development provides for furthercapacitive couplings to be provided in each case between the means forcapacitive signal extraction and the circuit arrangement for combiningthe extracted signals. The essential advantage of these furthercapacitive couplings resides in the potential isolation between themeans for capacitive signal extraction and the further circuitarrangement. An adaptation fault which is only present at one ignitionor at one ignition coil housing, for example a short-circuit to earth,does not influence the other detected signals. The further diagnosticdevice can be designed in such a way that the defective or incorrectlyconnected means for capacitive signal extraction can be identified.

An advantageous further development provides for at least one tuningelement to be arranged in the circuit arrangement for calibrating thelevel of the signal which can be tapped off at the output of the circuitarrangement. It is particularly advantageous if the individual means forcapacitive signal extraction are assigned tuning means for calibratingthe signal level. By this measure, the signals output by differentlydesigned means for capacitive signal extraction can be calibrated. Thesignals can then be compared directly with respect to the signal level.

In order to protect against high contact voltages, it is advantageous toprovide voltage limiters at the inputs of the circuit arrangement.

The means for capacitive signal extraction can be made easy to handle bydesigning them as plug-in components. It is expedient to provide plug-inconnections on the means, which connections permit rapid changing.

Further advantageous developments and improvements of the deviceaccording to the invention result from further subclaims in conjunctionwith the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an outline of a device according to the invention fordetecting signals in an ignition system; and

FIG. 2 shows an equivalent circuit diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an ignition system which contains two ignition coils 11,12. In each case one equivalent circuit diagram 13, 14 of the ignitioncoil 11, 12 is shown in the ignition coils 11, 12. Primary terminals 15,16; 17, 18 of the ignition coils 11, 12 lead to the ignition switchingdevices (not shown in FIG. 1). One secondary terminal 19, 20 of asecondary winding 21, 22 leads in each case to a high-voltage terminal23, 24.

In each case means 25, 26 for capacitive signal extraction are arrangedat the ignition coils 11, 12. The capacitive coupling is illustrated ineach case by means of a capacitor 27, 28. Since the capacitor 27, 28 isnot present as a separate component it has been shown in FIG. 1 bybroken lines.

The means 25 assigned to the ignition coil 11 is connected via a tuningarrangement 29 and a connecting line 30 and via a plug-in connection 31to a circuit arrangement 32 which has an output 33.

The means 26 assigned to the ignition coil 12 is connected via a plug-inconnection 34, a connecting line 35 and a plug-in connection 36 to thecircuit arrangement 32.

In FIG. 2 an equivalent circuit diagram of the device illustrateddiagrammatically in FIG. 1 is shown. The parts shown in FIG. 2 whichcoincide with the parts shown in FIG. 1 are provided with the samereference numerals as those in FIG. 1.

The circuit arrangement 32 contains voltage-limiting elements which arecomposed of in each case two diodes 37, 38; 39, 40. The voltage-limitingelements 37, 38; 39, 40 are each connected between input lines 41, 42 ofthe circuit arrangement 32 and ground 43. A resistor 44 and a capacitor45 are connected parallel to the output 33. The inputs 41, 42 areconnected to the output 33 via capacitors 46, 47 respectively. Theoutput 33 leads to a further diagnostic device 48.

The device according to the invention operates as follows:

The ignition system contains a plurality of ignition coils 11, 12 whichare provided for example for the successively timed ignition of aplurality of cylinders of an internal combustion engine. A conventionaldistributor is no longer present. In some ignition systems, connectinglines between the ignition coils 11, 12 and spark plugs (not shown inFIG. 1) are no longer accessible. Here, the spark plugs are connecteddirectly to the high-voltage terminals 23, 24. In order, nevertheless,to be able to perform high-voltage measurements on distributorlessignition systems with a plurality of ignition coils 11, 12, according tothe invention the means, 25, 26 for capacitive signal extraction areprovided. The means 25, 26 are preferably constructed as plug-in partswhich can be specially shaped for the purpose of adaptation to differentignition coils 11, 12. The capacitive coupling between the means 25, 26and the secondary windings 21, 22 of the ignition coils 11, 12 isillustrated by the coupling capacitor 27, 28 which is no longer presentas a separate component. Coupling only occurs to the primary side of theignition coils 11, 12. However, the voltages occurring in the secondarywinding 21, 22 are generally higher than those on the primary side by anorder of magnitude so that the voltage respectively coupled into themeans 25, 26 is always a measure of the voltage occurring on thesecondary side of the ignition coils 11, 12.

The signals which are capacitively extracted from the means 25, 26 arecombined in the circuit arrangement 32. The means 25, 26 can beconnected via connecting lines 30, 35 and via plug-in connections 31, 36to the arrangement 32. In a simpler embodiment, the plug-in connections31, 36 can be dispensed with.

The circuit diagram shown in FIG. 2 shows the combination of signals inthe circuit arrangement 32. In a very simple embodiment the individualsignals can be combined via a galvanic connection. However, capacitors46, 47 are preferably provided for potential isolating. This measureensures that incorrect adaptation of a means 25, 26 does not lead to theentire arrangement failing. Protection is provided in particular againsta short-circuit of one of the means 25, 26 to ground 43. The capacitor46, 47 avoid time-consuming searching for the means 25, 26 which isshort-circuited to ground. The diagnostic device 48 which furtheranalyses the signals combined at the output 33 easily permits theincorrectly connected means 25, 26 to be identified by reference to atime-related illustration of the signals.

In order to protect an operator against dangerous contact circuit, thevoltage arrangement 32 contains voltage-limiting means 37, 38; 39, 40.High-speed limiter diodes 37, 38; 39, 40, which are each connected atthe inputs 42, 41 to ground 43, are particularly suitable here. In FIG.2, the diodes 37, 38; 39, 40 are shown as series circuits with differentpolarities of the diodes 37, 38; 39, 40. In principle, other wiringconfigurations are also suitable.

In a different embodiment, the voltage-limiting means 37, 38; 39, 40 canbe provided in the tuning arrangement 29. With this arrangement of themeans 37, 38; 39, 40, dangerously high voltages can be suppressed nearto the source. Capacitive signal couplings in the subsequent circuitarrangement 32 and between lines 30, 35 are minimised.

Adaptation of the signal level or calibration of the entire arrangementis possible with the capacitor 45 which is constructed as a tuningelement with variable capacitance. The capacitor 45 forms, together withthe equivalent capacitors 27, 28 (shown in FIG. 1), in each case avoltage divider. The relatively high division ratio, which has to reducethe voltages lying in the kV range at the ignition coils 11, 12 tovalues which can be easily further processed, leads to a relatively highcapacitance value of the capacitor 45 so that the variable capacitanceof this capacitor 45 can be preferably simulated by parallel and/orseries circuits consisting of several capacitors.

The adaptation or calibration to the capacitor 45 has simultaneouseffects on the signals lying at the inputs 41, 42. An individualcalibration of the individual means 25, 26 is possible with the tuningarrangement 29 which is assigned to the corresponding means 25, 26. Thetuning arrangement 29 is joined constructionally for example to themeans 25, 26. It contains a matching circuit which is not shown ingreater detail in the figures and preferably also contains a capacitivevoltage divider. The tuning arrangement 29 is advantageously also to beaccommodated in the circuit arrangement 32.

A different embodiment provides for the use of plug-in connections 34 atthe means 25, 26. The tuning arrangement 29 here is preferablyintegrated in the plug-in connection 34. The plug-in connection 34permits the means 25, 26 to be changed rapidly.

The connecting lines 30, 35 present between the means 25, 26 and thecircuit arrangement 32 can be single-conductor lines. Preferably,shielded lines 30 which reduce the coupling-in of interference signalsare provided. The cable capacitance present in the shielded lines can beeasily taken into account during the calibration of the entirearrangement.

We claim:
 1. A device for detecting high-voltage characteristics in anignition system having at least two ignition coils, comprising:aplurality of first capacitive signal coupling means each being disposedon a respective one of the ignition coils for capacitively extracting avoltage signal from the ignition coil; a circuit arrangement connectedto said plurality of first capacitive signal coupling means andincluding an output, and means for combining the voltage signalsextracted from the ignition coils and producing a combined voltagesignal at the output; and a plurality of second capacitive signalcoupling means each connected between said means for combining and arespective one of the first capacitive signal coupling means forpotential isolation between the plurality of first capacitive signalcoupling means and a further portion of said circuit arrangement.
 2. Adevice as defined in claim 1, wherein said circuit arrangement includesa tuning element for calibrating the combined voltage at said output. 3.A device as defined in claim 1, further comprising a plurality of tuningarrangements each disposed between said means for combining and arespective one of said first capacitive signal coupling means forcalibrating the extracted voltage signal.
 4. A device as defined inclaim 2, further comprising a plurality of tuning arrangements eachdisposed between said means for combining and a respective one of saidfirst capacitive signal coupling means for calibrating the extractedvoltage signal.